The present invention relates to drive circuitry for matrix-addressed liquid crystal display (LCD) devices of the type responsive to the RMS levels of applied voltages.
LCD devices of the twisted-nematic type are used for displaying in seven-segment form selected ones of the decimal digits, for example. The twisted-nematic liquid crystal material is contained between parallel optical plates, one a linear polarizer and the other a linear analyzer. Without electric field, or potential gradient, applied in a direction normal to the plates, the twisted-nematic liquid crystal acts as a quarter-wave plate rotating polarization .pi./2 radians. Transparent electrodes on the inside surfaces of the confining plates are used to selectively apply electric field normal to those surfaces, responsive to which the twisted-nematic liquid crystal no longer rotates polarization. If the polarizer and analyzer are parallel-polarized, light transmission and absorption are respectively associated with application and non-application of electric field. If the polarizer and analyzer are cross-polarized, light absorption and transmission are respectively associated with application and non-application of electric field. A mirror may be used to back the analyzer to make the display device reflective rather than transmissive at the locations light absorption does not take place.
Conventional programmable LCD displays of the twisted-nematic type have a common or "back-plane" electrode for all portions of the display on one of the containing surfaces and a plurality of electrodes on the opposing containing surface, which segment "front-plane" electrodes can be selectively addressed with signal voltages to cause potential gradients, or electrical fields, between the common electrode and them. (It is possible to have the common electrode on the viewed surface of the LCD display and the segmented electrodes on the non-viewed surface, of course.) Such single-dimensional addressing undesirably requires as many address lines as programmable display segments.
Where the display comprises iterated display modules of information--e.g., where it is an array of programmable seven-segment decimal numerals--the number of address lines can be reduced by using two-dimensional, or matrix, addressing. The common or back-plane electrode is divided into one electrode per display module and corresponding segments opposing each module are parallelly addressed. An even number, 2n, of address lines can then select one from n.sup.2 display locations on a time-division-multiplexed basis, as compared to one from 2n display locations for single-dimensional addressing.
To maintain long lifetime of the twisted-nematic liquid crystal material it is desirable to avoid the applied electric field having a sustained direct component. This has led to the address lines in at least one of the dimensions used for matrix addressing being arranged to receive a ternary drive signal in prior art matrix addressing schemes. It is desirable to have a matrix-addressing scheme requiring only binary drive signals, to facilitate interfacing with conventional digital circuitry, such as a microprocessor, however.